Exercise to become hard when tender at one time (7)
I believe the answer is:
pesetas
'tender at one time' is the definition.
(former currency of Spain)
'exercise to become hard when' is the wordplay.
'exercise' becomes 'pe' (Physical Education).
'to become hard' becomes 'set' (set can mean to go hard).
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'pe'+'set'+'as'='PESETAS'
(Other definitions for pesetas that I've seen before include "Old units of Spanish currency before euros" , "Units of Spanish money" , "cash, once" , "Old Spanish money" , "old bread" .)